Thymosin .alpha..sub.1 is a heat stable, acidic polypeptide composed of 28 amino acid residues. This thymic hormone has been isolated from calf thymus thymosin fraction 5 and its amino acid sequence determined. Thymosin .alpha..sub.1 is one of several polypeptides present in thymosin fraction 5 which participate in the regulation, differentiation and function of thymic dependent lymphocytes (T-cells). The isolation, characterization and use of thymosin .alpha..sub.1 is described in greater detail in U.S. Pat. No. 4,079,127.
An immunoassay for a polypeptide hormone of the thymus known as thymopoietin or thymin is disclosed in U.S. Pat. No. 4,055,633. In particular, this patent discloses a radioimmunoassay for thymopoietin utilizing an antibody elicited by an immunogen comprising purified thymopoietin covalently coupled to an immunogenic carrier material such as bovine gamma globulin using glutaraldehyde as the coupling agent. The labelled antigen used in the assay is preferably .sup.125 I thymopoietin.
It should be noted that thymopoietin is totally non-analogous to thymosin .alpha..sub.1 in structure, amino acid composition and sequence, biological activity profile, physical properties and immunological properties.
A radioimmunoassay for a partially purified thymosin fraction, i.e. thymosin fraction 6, which is now known to contain a mixture of a number of polypeptides, is reported by Schulof et al., Fed. Proc. 32, 962 (1973). See also Goldstein et al. Fed. Proc. 33, 2053 (1974).